1. Field of the Invention
The present invention relates generally to radiation therapy devices, and more particularly, to a rotatable multi-element beam shaping device for use in a radiation therapy device.
2. Description of the Related Art
Conventional radiation therapy typically involves directing a radiation beam at a tumor in a patient according to deliver a predetermined dose of therapeutic radiation to the tumor according to an established treatment plan. This is typically accomplished using a radiation therapy device such as the device described in U.S. Pat. No. 5,668,847 issued Sep. 16, 1997 to Hernandez, the contents of which are incorporated herein for all purposes.
Tumors have three-dimensional treatment volumes which typically include segments of normal, healthy tissue and organs. Healthy tissue and organs are often in the treatment path of the radiation beam. This complicates treatment, because the healthy tissue and organs must be taken into account when delivering a dose of radiation to the tumor. While there is a need to minimize damage to healthy tissue and organs, there is an equally important need to ensure that the tumor receives an adequately high dose of radiation. Cure rates for many tumors are a sensitive function of the dose they receive. Therefore, it is important to closely match the radiation beam's shape and effects with the shape and volume of the tumor being treated.
In many radiation therapy devices, the treatment beam is projected through a pre-patient collimating device (a “collimator”), that defines the treatment beam profile or the treatment volume at the treatment zone. A number of different collimator techniques have been developed to attempt to conform the dose rate and the treatment volume to the shape of the tumor while taking nearby healthy tissue and organs into account. A first technique is to use a collimator with solid jaw blocks positioned along a path of the treatment beam to create a field shape based on the shape of the tumor to be treated. Typically, two sets of blocks are provided, including two blocks making up a Y-jaw generally disposed parallel to a Y-axis (with the Z-axis being parallel to the beam path), and two blocks making up an X-jaw generally disposed parallel to an X-axis. The X-jaw is conventionally placed between the Y-jaws and the patient.
These solid jaw blocks, however, do not provide sufficient variability in the field shape. In particular, where the tumor has a shape which requires a field edge relatively parallel to the edge of the jaw blocks, the edge of the jaw block becomes more predominant in forming the field edge. As a result, undulation of the field increases as well as the effective penumbra. This can be particularly difficult where the treatment beam is an X-ray beam. It is also difficult to adjust the field shape where the treatment beam is an electron beam due to electron attenuation and scattering.
Multileaf, or “multielement” block collimators were developed to provide more variation and control over the shape of the field at the treatment zone. An example multielement collimator is described in U.S. Pat. No. 5,591,983 issued to Hughes on Jan. 7, 1997. The Hughes collimator uses an X-jaw which has two blocks each made up of a number of individual elements. Each of the elements of the X-jaw can be moved longitudinally across the path of the radiation beam to create a desired beam shape at the point of treatment.
Further control over the shape of the beam is desirable. Existing block collimators fix the position of the Y-jaw with respect to the X-jaw, that is, the Y-jaw blocks do not move independently of the X-jaw blocks. As a result, many treatments involve tumors having a shape relatively parallel to one or more edges of either the X- or Y-jaws. Delivery of an appropriate therapeutic dose of radiation to these tumors can be difficult due to radiation scattering, undulation, and penumbra effects.
Therefore, it would be desirable to provide a system and method which allows further control of each of the blocks, each of the jaws, and each of the elements in a multielement collimator to increase control over the beam shape, including control over the penumbra and undulation effects which can arise when the treatment field edge becomes relatively parallel to edges of the blocks, jaws, and/or elements of the collimator. It would also be desirable to provide control over the beam shape to provide more accurate control during treatment to accommodate beam attenuation and scattering.